Theory of superconductivity in multilayer systems

Superconducting properties of artificial multilayer superconductors and layered cuprate-oxide superconductors are studied on a unified basis of the proximity effect. In multilayers composed of two superconductors with different H_c2, an anomaly in the temperature dependence of H_c2 is predicted in relation to a transition between the two different vortex states. For the cuprate oxides we take a model in which superconductivity is generated in the CuO₂ layers and superconductivity in the other layers is induced by the proximity effect. On a basis of this model, anomalous behavior of the tunneling conductance, the optical conductivity, the critical fields, the critical current, etc. observed in the superconducting state of the oxides is explained.